Utility interactive inverter systems have been developed for converting DC power generated by e.g. a solar battery into AC power to supply a three-phase power system with the AC power. A utility interactive inverter system generally employs a full-bridge three-phase inverter circuit. The full-bridge three-phase inverter circuit includes a bridge circuit constituted of three sets of serially connected two switching elements (hereinafter, such set will be referred to as “arm”) connected in parallel, and is configured so as to obtain three outputs corresponding to U-phase, V-phase, and W-phase from the connection point between the switching elements of each of the arms. The three-phase inverter circuit converts each of phase voltage control signals (sine wave voltage signals of a phase shifted by 2π/3 from each other), representing a target output of each phase, into a PWM signal, and controls the on/off action of the switching elements of each arm of the inverter circuit with the PWM signal, thereby controlling an AC voltage signal to be output from the inverter circuit to U-phase, V-phase, and W-phase, respectively.
FIG. 22 is a block diagram for explaining an example of a utility interactive inverter system including a conventional inverter control circuit.
In the figure, the inverter circuit 2 in the inverter system A′ is constituted of a full-bridge three-phase inverter circuit. To each arm of the bridge circuit in the inverter circuit 2, a DC voltage which is inputted from a DC power source 1 is applied. To the six switching elements in total, which consists of two switching elements provided in each arm, six PWM signals inputted from an inverter control circuit 6′ are respectively input. The PWM signals control the on/off action of the six switching elements, so that the inverter circuit 2 outputs pulse AC voltage signals corresponding to U-phase, V-phase, and W-phase, respectively.
In FIG. 22, the signal line from the inverter control circuit 6′ to the inverter circuit 2 is drawn with six oblique lines, which indicate the number of signal lines for the PWM signal. Thus, FIG. 22 means that six PWM signals are input to the inverter circuit 2 from the inverter control circuit 6′ for the six switching elements.
The filter circuit 3 eliminates high-frequency components such as a switching noise from the three pulse AC voltage signals outputted from the inverter circuit 2. thereby converting the pulse AC voltage signals into sine wave-shaped AC voltage signals. Then the amplitude of the three sine wave AC voltage signals (corresponding to U-phase, V-phase, and W-phase) is adjusted by a transformer circuit 4, to be outputted to the corresponding phase in a system 5. In the inverter system A′, each of the three phase voltage signals outputted from the transformer circuit 4 to the system 5 need to be matched with the AC voltage signal of the corresponding phase of the system 5. For this purpose, the inverter control circuit 6′ generates the phase voltage control signal of the phases for representing the target output basically in accordance with the phase voltage signal of each phase of the system 5, to generate a PWM signal PSu by comparing the phase voltage control signal Su with a predetermined carrier signal (triangle wave signal) Sc, as shown in FIG. 23.
FIG. 23 illustrates the principle of generating the PWM signal PSu for a switching element constituting the arm corresponding to U-phase in the inverter circuit 2, by comparing the phase voltage control signal Su of U-phase with the carrier signal Sc, assuming the phase of U-phase as the reference. Since two switching elements are serially connected in the arm corresponding to U-phase in the inverter circuit 2, the PWM signal PSu shown in the figure is utilized as the PWM signal for one of the switching elements, and a PWM signal having the inverted level of the PWM signal PSu is utilized as the PWM signal for the other switching element.
Also, if it is supposed that the phase of the phase voltage control signal of V-phase is advanced by 2π/3 from the phase voltage control signal Su of U-phase, a pattern waveform of a PWM signal PSv for the switching elements of the arm corresponding to V-phase is advanced by 2π/3 from that of the PWM signal PSu. If it is supposed that the phase voltage control signal for W-phase is advanced in phase by 4π/3 from the phase voltage control signal Su for U-phase, a pattern waveform of a PWM signal PSw for the switching elements of the arm corresponding to W-phase is advanced by 4π/3 from that of the PWM signal PSu.
FIG. 23 illustrates the principle of generation of the PWM signal based on what is known as a triangle wave comparison method. Specifically, the level range Dc (peak-to-peak value of the amplitude) of the carrier signal Sc is set larger than the level range Du (peak-to-peak value of the amplitude) of the phase voltage control signal Su, and the level of the carrier signal Sc and that of the phase voltage control signal Su are compared, thereby generating the PWM signal PSu having a duty ratio of the pulse signal variably determined in accordance with the positive and negative amplitude values of the phase voltage control signal Su.
As is known, in the PWM signal for the bridge type inverter circuit, a dead time has to be provided so as to prevent the pair of switching elements of each arm from being simultaneously turned on (and to prevent a short-circuit of the arm), and hence the level range Dc of the carrier signal Sc has to be given a certain margin. Accordingly, the level range Dc of the carrier signal Sc has to be provided with an variation region, and the phase voltage control signal Su has to be fluctuated within the variation region.
As shown in FIG. 24, if a third-order harmonic is superimposed on the phase voltage control signal, the peak-to-peak value Pu0 of the phase voltage control signal becomes smaller than a peak-to-peak value (Pu) without the third-order harmonic superposed (in FIG. 24, a waveform Vu represents the phase voltage control signal on which the third-order harmonic is not superposed, and a waveform Vu0 represents the phase voltage control signal with the third-order harmonic superposed thereon). In the case where the phase voltage signals of U-phase, V-phase and W-phase are balanced, even though third-order harmonic is included in each phase voltage signal, the third-order harmonic is not included in the line voltage signal between the U-V, V-W, and W-U lines (See waveform Xuv in FIG. 24), and hence is not outputted to the system 5. Thus, the inverter control circuit 6′ is configured so as to enhance the voltage utilization efficiency of the carrier signal Sc in the PWM signal generation process based on the triangle wave comparison method, by utilizing the phase voltage control signal with the third-order harmonic superposed thereon, as the target output of the respective phases.
In FIG. 22, the inverter control circuit 6′ includes a target value signal generation circuit 61′ and a PWM signal generation circuit 62′. The target value signal generation circuit 61′ serves to generate the phase voltage control signal of the respective phases with the third-order harmonic superposed thereon as a target output signal (hereinafter, “target value signal”). The PWM signal generation circuit 62′ serves to generate the six PWM signals based on the triangle wave comparison method, utilizing the target value signals Xu0, Xv0, Xw0 of the respective phases input from the target value signal generation circuit 61′.
Also, a phase voltage control signal generation circuit 611′ in the target value signal generation circuit 61′ serves to generate the phase voltage control signals of the respective phases. A third-order harmonic superposing circuit 612′ serves to superpose the third-order harmonic on each of the phase voltage control signals Xu, Xv, Xw of the respective phases outputted by the phase voltage control signal generation circuit 611′, thereby generating the target value signals Xu0, Xv0, Xw0. It should also be noted that the phase voltage control signal generation circuit 611′ utilizes the output voltage (DC) from the DC power source 1 detected by a DC voltage sensor 7, the phase current (AC) running through each phase of the system 5 detected by a current sensor 8, and the line voltage (AC) of the system 5 detected by a line voltage sensor 9, so as to generate the phase voltage control signals Xu, Xv, Xw of the respective phases, in order to match the target output with the actual phase voltage signals of the respective phases of the system 5.
In the utility interactive inverter system, since the DC power is converted into the AC power by the on/off action of the switching elements in the inverter circuit 2, the power consumption for the on/off action of the switching elements causes the power conversion loss (generally called “switching loss”). Accordingly, for the purpose of improving the power conversion efficiency in the inverter circuit 2, it has been proposed to reduce the frequency of the carrier signal in the PWM signal generation circuit 62′ and to switching the frequency for reducing the switching loss.
Patent Document 1: JP-A-2007-228745